The Fischer-Tropsch process generally comprises a first step which consists in reacting a source of carbon (such as coal, natural gas or biomass) with a source of oxygen (such as steam, air or oxygen) to form a mixture of carbon monoxide and hydrogen, usually referred to as syngas. A second step is then carried out, which involves contacting the syngas with a Fischer-Tropsch catalyst, which leads to hydrocarbons and water. The main products of the Fischer-Tropsch reaction are linear olefins and paraffins and water. It is well known that the nature of the hydrocarbons produced, and their chain length, may vary depending on the process conditions and the catalyst used. The third step involves isomerisation of the hydrocarbons formed in the second step to produce more valuable products. For instance, the longer chains in the product may be cracked to form products in the diesel or gasoline range, and linear paraffins may be isomerised to improve diesel product properties such as cloud point and pour point. Generally, adapted hydrotreating catalysts are used for this third step.
Typical catalysts used in the second step of the above process are made by kneading or impregnation of carriers made of micrometric particles of alumina, silica, titania, silicon carbide, carbon or mixtures thereof, with a metal such as cobalt having a particle size comprised between 10 and 20 nm.
They suffer from the drawback that most of the catalytic metal, which is inside of the catalyst particles, does not participate in the catalytic process. There is thus a waste of the catalytic metal which is economically disadvantageous, especially in the case of cobalt, which is a rather expensive catalytic metal. In order to overcome this drawback, the present inventors have contemplated using core-shell particles, which comprise a core in a cheaper material than cobalt and a shell of cobalt. These particles comprise a much lower amount of cobalt than known catalysts, while approaching the same catalytic activity.
A cobalt-based catalyst, comprising particles having a core-shell structure and which is said to be selective towards the Fischer-Tropsch reaction, has been disclosed in U.S. Pat. No. 7,361,626. This catalyst may be prepared according to the following process. A zinc oxide layer is first applied on the surface of an oxidic core material, typically, alumina, by means of a so called “layer-by-layer” (or “LBL”) method, so as to obtain a core-shell support. A catalytically active material, such as cobalt, is then added to this core-shell support either by impregnation or deposition-precipitation. Thereafter, the resulting particles are calcined and hydrogenated to produce a metal-based catalyst. This process involves several steps and the use of a surfactant to anchor the zinc oxide layer to the chemically inert aluminium core, which increases its cost. Moreover, the core-shell particles thus obtained can be described as having a core-shell support coated with large crystallites of cobalt. Consequently, they do not allow reducing the cobalt content of the catalyst particles, which is not the purpose for which these attrition-resistant particles have been designed. Moreover, the size of these particles may be detrimental to the activity and selectivity of the catalyst made therefrom.
Therefore, there remains the need to provide a simple and cost-effective method for preparing core-shell particles which dimensions may be easily controlled and which may be used to manufacture a catalyst having good selectivity towards the Fischer-Tropsch reaction and good productivity.
This need has been satisfied by a novel method which involves homogeneous deposition-precipitation of cobalt carbonate onto nanoparticles of alumina. This method leads to specific nanoparticles having a core consisting of nanoparticles of alumina and a shell comprising cobalt. To the inventors' knowledge, these particles have never been described before. The precipitation-deposition method has already been applied in U.S. Pat. No. 7,851,404 and US 2007/270514 to the manufacture of cobalt-based catalysts. In these documents, a cobalt compound, obtained by decomposition of a cobalt amine complex under basic conditions, is deposited onto particles of a carrier material in the form of a powder or of a shaped granular material, or onto a titania-coated alumina. The carrier core has a mean diameter of several microns and no information is given about cobalt crystallite size. The thickness of the cobalt layer in US 2007/270514 ranges from 5 to 250 μm. It has been shown in the Examples below that the productivity, on cobalt mass basis, of these catalysts could still be improved.
Other core-shell nanoparticles comprising a core of a carrier material selected from iron oxide, copper and silicon dioxide have been disclosed in EP 2 530 125, Nachal D. Subramanian et al., Catalysis Science & Technology, Vol.2, No. 3, January 2012 and in CN 101 954 256, respectively. In the second one of these documents, the core-shell particles need to be oxidized so as to remove the surfactants used in their synthesis and bound to the surface of the nanoparticles. The resulting nanoparticles show a polyhedron-like morphology with some diffusion of copper from the core to the surface of the nanoparticles.